Allow not to set varname

tzis/tzis.py

@@ -796,6 +796,13 @@ class Tzis:
         maxretries=3,
         trusted=True,
     ):
+        if not self.mf_dset :
+            raise "Do not know what to write. Please define object mf_dset first."
+        
+        if not self.varname:
+            if self.verbose:
+                print("You did not specify a varname. Take the first data var out of the dset.")
+            self.varname = self._get_varname(self.mf_dset)            
         pargs = locals()
         writestore = self.set_write_store(self.store, self.fsspec_store)